Pool water balance calculator (CSI and LSI)

The Calcite Saturation Index (CSI) tells you whether water will dissolve plaster (negative) or deposit scale (positive). Aim for −0.3 to +0.3. Water at pH 7.5, TA 80, calcium 300 and CYA 40 at 81 °F has a CSI of -0.06.

CSI / LSI calculator

−0.07

Calcite Saturation Index (CSI)

Classic LSI −0.15

Balanced.

−1 corrosivescaling +1

Aim for −0.3 to +0.3. Plaster pools do best slightly below zero to just above.
No change needed.

Same water at other pH values
At this pH	CSI
pH 7.2	−0.35
pH 7.4	−0.16
pH 7.6	+0.03
pH 7.8	+0.22

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CSI and LSI

The Langelier index (LSI) was designed for drinking water. Pools have cyanuric acid and sometimes borates, which show up in the alkalinity test but do nothing for calcium balance. CSI corrects for them and uses proper activity and temperature terms, so it is the better number for pools. Ignoring CYA, the same water would read -0.07 on a simple LSI.

What moves it

pH has the biggest effect: +0.1 pH is roughly +0.1 CSI.
Calcium hardness: doubling it adds about +0.3.
Alkalinity: doubling it adds about +0.3.
Temperature: warm water is more scale forming. A hot spa scales sooner than the pool.

Questions

What is a good CSI for a pool?

Between −0.3 and +0.3. Plaster pools do best around −0.3 to 0; vinyl and fiberglass pools are less sensitive.

Is CSI the same as LSI?

They measure the same thing. CSI corrects for cyanuric acid and borates, which matters in pools.